VoLTE Roaming – Critical enabler or long term ideal?

Voice roaming, the ability to just switch your mobile on in almost any country and make/receive calls using your own phone number, was a revolutionary invention in the 1990s with GSM. The advent of 3G continued the convenience and added medium speed data. LTE data roaming increased the speed further.

But end-to-end voice roaming using LTE is extremely rare. Will this hold back deployment of LTE only networks including in-building systems at 3.5GHz (CBRS) or 5GHz (MulteFire)?

A quick recap of VoLTE deployment status

Of the 500 live LTE networks today, around 90% rely on 3G (or 2G) for voice calls using CSFB (Circuit Switched Fall Back). It works well where there is coverage but causes issues where LTE coverage (including in-building penetration) exceeds that of 2G/3G.

As of January 2016, 46 networks had deployed VoLTE today and the number continues to grow. Take-up can be high - T-Mobile USA report 52% of their calls use VoLTE. It allows end-to-end calls between two VoLTE capable handsets on the same home network, with fast call setup and the option to add video calls. But the primary reasons for VoLTE deployment has been to make more efficient use of available spectrum (such as Three UK’s SuperVoice using 800MHz band to dramatically improve coverage and in-building penetration) or legacy technology (Verizon USA migrating away from CDMA). There is little extra revenue to be made from voice service today and this has held back investment for VoLTE as a service in its own right.

VoLTE Interconnection

VoLTE interconnection, which would allow end-to-end VoLTE calls between users of different networks, is still at a very early stage. This requires a peer-to-peer connection, parallel to the existing circuit switched landline interconnect, with issues around billing, numbering, security to be considered. The first international VoLTE call took place only a year ago. Direct interconnect between Verizon and AT&T is operational, but only in selected areas.

In the short term, most operators seem content with downgrading VoLTE calls to circuit switched when interworking with other networks. This may lead to a call being transcoded several times – originating as VoLTE, interconnected as circuit switched call, then converted into VoLTE in the destination network. It negates key benefits of VoLTE such as very fast call setup, HD voice or video.

HD voice calling (using the same higher quality codec as VoLTE) is also possible using 3G. This could be used to provide end-to-end HD voice between 3G and VoLTE devices. The question is whether operators see this as a worthwhile investment for calls connected via partner networks. They may prefer to focus on ensuring higher quality for their own subscribers on-net.

“Circuit Switched” VoLTE Roaming

Being able to use VoLTE when visiting a foreign network could involve considerably more complexity than first appears.

The simplest method is to roam using existing 2G/3G circuit switched between networks, converting the call within the visited and/or home network into VoLTE. While this wouldn’t benefit from HD voice or fast call setup, it would allow the call to be delivered via LTE – likely providing better coverage, quality and spectrum efficiency. CSFB (fallback to 2G/3G) would also work and avoid coverage blackspot issues.

Many VoLTE operators restrict this service to a limited number of smartphone models and software releases. For example, Three UK will support any iPhone (even if not bought from them) but not on any non-Three devices even if the manufacturer states they are VoLTE compatible. This is partly due to configuration and provisioning issues of carrier settings.

The net result is that visitors to the UK could benefit from Three’s 800MHz coverage with calls delivered through VoLTE. They may notice faster call setup and HD Voice for calls within Three’s network, but would revert back to normal quality for calls with other networks.

End-to-end VoLTE roaming

There are two main architecture choices to facilitate end-to-end VoLTE calls when roaming.

Home Routing: Handle the call entirely from the home network through a dumb data pipe. The visited network has no knowledge of the call – it’s just another data stream. This makes it difficult to deal with emergency calls or special local numbers. Fallback to 2G/3G wouldn’t be feasible – it’s all or nothing on LTE only.

SIP Proxy: Peer-to-peer VoLTE call management, delegating call control to the visited network. This is similar to the mechanisms used for 2G/3G roaming, operating in a parallel using packet switched protocols based on SIP.

Option (1) may not meet the regulatory requirements of many countries and may provide poor performance until LTE is ubiquitous. This is similar to many OTT voice and video services (Skype, Viber, Facetime etc.).

Option (2) involves considerable investment in testing and configuration. During this transition, there would be multiple roaming solutions to be supported.

Carriers also have security/privacy concerns around exposing their ENUM database to many third parties.

However, end users might argue they should receive the same enhanced communication service when at home or abroad. This may also become an expectation when roaming onto neutral host enterprise networks.

A roadmap for seamless service

It may well be feasible for 2G/3G only networks to allow their subscribers to roam onto LTE only networks using existing SIM cards and roaming protocols. The visited network would appear as “just another” roamed-to network, with the same signalling and billing messages. Internally, it can process and deliver voice and data entirely over LTE if it chooses to (and the smartphones are capable of it).

This should not hold back deployment of new LTE-only in-building networks including those using 3.5GHz or 5GHz bands.

But for those wanting to improve voice service, the following steps provide a logical approach to VoLTE deployment evolution.